In many roofing applications, for example in large, flat commercial roof decks, the roofing substrate is a concrete, light weight concrete, wood, gypsum, wood fiber or steel roof deck. The roofing membrane is used to seal and protect the roof deck from environmental weather conditions and is placed over insulation boards, which provide insulative qualities. The insulation boards are typically secured to the roofing substrate or roof deck via an adhesive composition or fasteners. The roofing membrane may be made of various materials, such as polymeric materials including EPDM (ethylene propylene diene M-rubber), Mod Bit (Modified Bitumen), TPO (thermoplastic polyolefin), or polyvinyl chloride (PVC). The roofing membrane may also be a composite material that includes EPDM or TPO. The roofing membrane is adhered overtop insulation boards or panels using an adhesive composition such as mopping asphalt (typically Type III or Type IV) or other conventional adhesive compositions. Conventional adhesives normally are required to be applied to both the roofing membrane and the substrate.
A conventional adhesive composition used to adhere the roofing membrane to the roof deck or other substrate includes the use of a solvent based elastomer, such as, for example, a polychloroprene rubber in an acetone or toluene solvent. However, typical polychloroprene rubber adhesives are solvent based and contain high (>250 g/L) levels of volatile organic compounds (VOC). These adhesives cannot be sold in areas where federal, state or local regulations prohibit the use of such high VOC adhesives. Other solvent based adhesives use VOC exempt solvents like acetone, t-butyl acetate and para-chlorobenzotrifluoride which will work in their intended applications, but they are slow to evaporate at cold temperatures or have very low flash point. Other adhesives are based on waterborne emulsions of polychloroprene rubber However, while useful for their intended purpose, these water based adhesives may have issues regarding temperature restrictions, long curing times, odor concerns and freeze-thaw stability. In addition, these adhesives are contact adhesives that require full coverage between the substrate and the membrane. Accordingly, solvent and water based elastomer adhesives must be applied to both the substrate and the roofing membrane and cover substantially the entire surfaces of the adhering components. These adhesives can blister when used between two non-breathable surfaces due to the incomplete evaporation of solvent or water from the above adhesives. For example, blistering may occur on a new roof membrane when applied over an existing roof membrane in recover applications.
Traditional one-part polyurethane adhesives, such as in U.S. Pat. No. 6,679,018 issued to Georgeau, are limited by their need for ambient moisture to cure. Therefore these moisture cure adhesives have limited utility in low humidity environments or when used in between two moisture impermeable membranes, for example, between a new roof membrane and an existing roof membrane in recover applications.
Traditional two-part membrane adhesives, such as U.S. Pat. No. 7,622,187 issued to Clarke and U.S. Pat. Nos. 6,938,386 and 6,742,313 issued to Ritland, require a modified membrane surface to attach the membrane to a roofing substrate. For example, the surface modified membrane is typically an EPDM or TPO incorporating a “fleece” backing.
Other polyurethane adhesive compositions, such as the adhesive composition disclosed in U.S. Pat. No. 5,872,203 issued to Wen et al., utilize compositions limited to a ratio of isocyanate groups to hydroxyl groups of less than 1.5:1. Additionally, Wen discloses the use of organosilanes as adhesion promoters. Organosilanes are known to improve initial adhesion, but have no effect on heat aging properties.
Accordingly, there is room in the art for adhesive compositions in roofing applications that exhibit favorable properties, such as sufficient adhesive strength, shelf life, cure time, tack, that are easily applied, that have low VOC content and are 100% solids with essentially no volatility.